FIG. 1a shows a cross-sectional view of a portion of an electronic device 100, for example a semiconductor device, comprising die support 102, electronic die (e.g., semiconductor chip) 104, connector, or clip, 106 and a lead 108 of a leadframe.
Die 104 is attached to die support 102 by bonding material 110. Contact 112 on die 104 is bonded to a die-end 114 of connector 106 by an electrically conductive bonding material 116, such as, for example, solder, sinter materials, conductive glue, or the like.
A lead-end 118 of connector 106 is bonded to a connector-pad 120, which comprises a planar region of lead 108, by electrically conductive bonding material 122 that may, again, comprise solder, sinter materials, conductive glue, or the like.
A portion of the lead-end 118 of connector 106 adjacent the connector-pad 120 is generally flat.
In electronic devices, it is essential that a solder joint does not comprise a so-called “dry joint” or “cold-solder joint” so as to ensure good electrical conduction between joined parts. Any defect in a solder joint can lead to a joint that is non-functional either immediately or after a period of use. Ideally a surface at a solder-atmosphere interface of a fillet of solder formed around a region where two parts to be joined meet is concave. Presence of a concave surface can reduce stress concentration at the surface of a solder joint, which may reduce the possibility of cracks forming in the solder and propagating through the solder.
Techniques have been developed to mimic the stresses that a solder joint will undergo during a period of use and these techniques employ repeated stressing and de-stressing of a component to induce failure through fatigue. One known example comprises temperature cycle testing, which comprises a process of cycling through temperature extremes, typically at relatively high rates of change to evaluate product reliability.
In the prior art arrangement illustrated in FIG. 1a, a concave surface of electrically conductive bonding material 122 is formed at a first end 124a of the connector-connector pad interface. However, a first article inspection revealed that a similar shaped surface is not formed at a second end 124b of the connector-connector pad interface, which is influenced by the shape of the end of the connector 16. That is, the joint is not symmetric.
In a thermal cycling test (TCT) of a joint of electrically conductive bonding material 122 as illustrated, the joint failed after 1000 cycles due to a mismatch in the coefficients of thermal expansion of the electrically conductive bonding material, e.g. an epoxy moulding compound, and the connector-pad 120 of lead 108. Failure was caused by a crack, which formed in the electrically conductive bonding material 122 at the second end 124b of the connector-connector pad interface, propagating through the electrically conductive bonding material 122.
To mitigate against the possibility of solder crack occurring in a solder joint arrangement such as that illustrated in FIG. 1a, a die-attach and clip-attach (DACA) process is optimised to provide a greater volume of solder on the connector-pad. However, possibility of solder crack occurring is still limited by shape and/or configuration of an end of connector 106. In a solder joint formed using such an optimised process, joint failure occurred after 2000 cycles in a TCT.
The present invention has been devised with the foregoing in mind.